Actuator

ABSTRACT

The present invention provides an actuator, comprising a fiber and a temperature regulator capable of at least one of heating and cooling the fiber. The fiber is twisted around a longitudinal axis thereof. The fiber is folded so as to have a shape of a cylindrical coil. The fiber is formed of linear low-density polyethylene. The following mathematical formula (I) is satisfied: D/d&lt;1 (I), where D represents a mean diameter of the cylindrical coil; and d represents a diameter of the fiber.

BACKGROUND

1. Technical Field

The present invention relates to an actuator.

2. Description of the Rerated Art

United States Pre-Grant Patent Application Publication No. 2015/0219078discloses an actuator formed of a polymer fiber. In United StatesPre-Grant Patent Application Publication No. 2015/0219078, the polymerfiber is twisted and folded. United States Pre-Grant Patent ApplicationPublication No. 2015/0219078 is incorporated herewith by reference.

SUMMARY

An object of the present invention is to provide an actuator having ahigh displacement rate.

The present invention provides an actuator, comprising:

-   -   a fiber; and    -   a temperature regulator capable of at least one of heating and        cooling the fiber, wherein    -   the fiber is twisted around a longitudinal axis thereof;    -   the fiber is folded so as to have a shape of a cylindrical coil;    -   the fiber is formed of linear low-density polyethylene; and    -   the following mathematical formula (I) is satisfied:

D/d<1  (I)

-   -   where    -   D represents a mean diameter of the cylindrical coil; and    -   d represents a diameter of the fiber.

The spirits of the present invention includes a method for extending andcontracting a fiber; the method comprising:

-   -   (a) heating the fiber to contract the fiber; wherein    -   the fiber is twisted around a longitudinal axis thereof;    -   the fiber is folded so as to have a shape of a cylindrical coil;    -   the fiber is formed of linear low-density polyethylene;    -   the following mathematical formula (I) is satisfied:

D/d<1  (I)

-   -   where    -   D represents a mean diameter of the cylindrical coil; and    -   d represents a diameter of the fiber; and    -   the fiber is contracted along an axis direction of the        cylindrical coil; and    -   (b) cooling the fiber to extend the fiber; wherein    -   the fiber is extended along the axis direction of the        cylindrical coil.

The present invention provides an actuator having a high displacementrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross-sectional view of an actuator according to thepresent embodiment.

FIG. 1B shows a cross-sectional view of the actuator in the state wherea fiber is contracted.

FIG. 2A shows a schematic view of a fiber 111 a which is neither twistednor folded.

FIG. 2B shows a schematic view of a fiber 111 b which is twisted,however, is not folded.

FIG. 2C shows a schematic view of a fiber 111 c which is twisted andfolded.

FIG. 2D shows a schematic view of a cylindrical coil formed of thefolded fiber 111 c.

FIG. 2E shows a schematic view of the fiber 111 c coated with a metalfilm 140.

FIG. 3 shows a schematic view of a state of the fiber 111 c before thefiber 111 c is heated.

FIG. 4 shows a schematic view of a state of the fiber 111 c after thefiber 111 c is heated.

FIG. 5A is a photograph showing the extended fiber 111 a obtained in theinventive example 1.

FIG. 5B is a photograph showing the extended fiber 111 a obtained in thecomparative example 1A.

FIG. 5C is a photograph showing the extended fiber 111 a obtained in thecomparative example 2A.

FIG. 5D is a photograph showing the extended fiber 111 a obtained in thecomparative example 3A.

FIG. 6A is a photograph showing the folded fiber 111 c obtained in theinventive example 1A.

FIG. 6B is a photograph showing the folded fiber 111 c obtained in thecomparative example 2A.

FIG. 6C is a photograph showing the folded fiber 111 c obtained in thecomparative example 3A.

FIG. 7 is a graph showing thermomechanical properties of the fibers 111c obtained in the inventive example 1A, the comparative example 2A andthe comparative example 3A.

DETAILED DESCRIPTION OF THE EMBODIMENT

The embodiment of the present invention will be described with referenceto the drawings.

(Terms)

First, the reference numbers added to the term “fiber” in the instantspecification will be defined as below.

The term “fiber 111 a” means a fiber which is neither twisted norfolded. See FIG. 2A. The fiber 111 a may be referred to as “extendedfiber 111 a”.

The term “fiber 111 b” means a fiber which is twisted, however, is notfolded. See FIG. 2B. The fiber 111 b may be referred to as “twistedfiber 111 b”.

The term “fiber 111 c” means a fiber which is twisted and folded. SeeFIG. 2C. The fiber 111 c may be referred to as “folded fiber 111 c”.

The term “fiber 111” comprehensively includes the fibers 111 a-111 c.

In the instant specification, there is not a fiber which is folded,however, is not twisted.

Embodiment

As shown in FIG. 1A, an actuator according to the present embodimentcomprises a fiber 111 c consisting of linear low-density polyethyleneand a temperature regulator 130. The temperature regulator 130 iscapable of at least one of heating and cooling the fiber 111 c.

(Fiber)

The fiber 111 c is twisted around the longitudinal axis thereof. Thefiber 111 c is folded so as to have a helix shape. In other words, thefiber 111 c is folded so as to have a shape of a cylindrical coil.

First, a method for fabricating the fiber 111 c used in the presentembodiment will be described with reference to FIG. 2A FIG. 2C.

As shown in FIG. 2A, a fiber 111 a having a length of L1 and a diameterof d is prepared. Needless to say, the fiber 111 a is elongate and has afiber axis 111LA. In FIG. 2A, the fiber 111 a is neither twisted norfolded yet. The fiber axis 111LA is also a central axis of the fiber 111a and parallel to the x-axis direction.

Next, as shown in FIG. 2B, the fiber 111 a is twisted. In this way, thefiber 111 b is obtained. More specifically, one end of the fiber 111 ais twisted around the fiber axis 111LA, while the other end of the fiber111 a is fixed so as not to be twisted around the fiber axis 111LA. Inthis way, the twisted fiber 111 b is obtained. In FIG. 2B, the fiber 111b is twisted, however, is not yet folded. The fiber 111 b has a lengthof L2. The fiber 111 b has a diameter d′ which is slightly greater thanthe diameter d. The fiber axis 111LA is parallel to the x-axisdirection. The value of L2 is equal to or less than the value of L1.

The above-mentioned twists are continued in such a way that the one endof the fiber 111 b is rotated many times around the fiber axis 111LA. Asa result, as shown in FIG. 2C, the fiber 111 is folded while beingrotated. More specifically, the fiber 111 is folded so as to have alength of L3 which is smaller than L1 and to have a mean diameter of Dwhich is more than d. Also in this stage, the other end of the fiber 111is fixed so as not to be twisted around the fiber axis 111LA. In thisway, the fiber 111 c which is twisted and folded is obtained. An angleα_(f) shown in FIG. 2B represents a fiber bias angle. The fiber biasangle α_(f) is a twist angle of the fiber 111 with regard to the fiberaxis 111LA. The mean diameter D is obtained by subtracting the diameterd of the fiber from the external diameter D′ of the cylindrical coil.

As shown in FIG. 2C, after the fiber 111 is folded, the fiber axis 111LAis no longer parallel to the x axis. The folded fiber 111 c has a helixshape. In other words, the folded fiber 111 c has a shape of acylindrical coil. In other words, the folded fiber 111 c has a shape ofa spring. As shown in FIG. 2C, the coil has a pitch of p. The pitch p isequal to one period of the coil. See FIG. 2D.

As shown in FIG. 2D, the rotation direction R1 of the helix (i.e., therotation direction R1 of the cylindrical coil) accords with the fiberaxis iii LA of the folded fiber 111 c . Needless to say, when the fiber111 is twisted clockwise around the fiber axis 111LA in FIG. 2B, thefiber 111 is folded with rotating clockwise in FIG. 2C. Similarly, whenthe fiber 111 is twisted counterclockwise around the fiber axis 111LA inFIG. 2B, the fiber 111 is folded with rotating counterclockwise in FIG.2C.

The cylindrical coil formed of the folded fiber 111 c has a meandiameter of D. The cylindrical coil has a longitudinal axis 110LA.Hereinafter, the longitudinal axis 110LA of the cylindrical coil isreferred to as a coil axis 110LA.

An angle α_(c) shown in FIG. 2C represents a coil bias angle. The coilbias angle α_(c) is formed between a plane perpendicular to the coilaxis 110LA and the fiber axis 111LA of the folded fiber 111 c.

In the present embodiment, the fiber 111 is formed of linear low-densitypolyethylene (hereinafter, referred to as “L-LDPE”). Since the fiber 111is formed of linear low-density polyethylene, the folded fiber 111 c hasa spring index C of less than 1.

As well known, the spring index C is represented by the followingmathematical formula (I):

C=D/d

-   -   where    -   D represents a mean diameter of the cylindrical coil formed of        the folded fiber 111 c, and    -   d represents a diameter of the fiber 111.

It gets harder to extend the cylindrical coil with a decrease in thespring index C. In other words, the amount of the extension of thecylindrical coil is smaller with a decrease in the spring index C, in acase where a force F applied to the cylindrical coil along the axisdirection (i.e., the longitudinal direction) of the cylindrical coil isconstant.

On the other hand, the cylindrical coil is extended easily with anincrease in the spring index C. In other words, the amount of theextension of the cylindrical coil is greater with an increase in thespring index C, in a case where a force F applied to the cylindricalcoil along the axis direction (i.e., the longitudinal direction) of thecylindrical coil is constant.

Therefore, a cylindrical coil having a high spring index C is “soft” anda cylindrical coil having a low spring index C is “stiff”. When thenumber of the twist of the fiber 111 around the fiber axis 111LA isincreased, namely, when the number of the rotation of the fiber 111around the fiber axis 111LA is increased, the spring index C of theobtained cylindrical coil is decreased. However, when the number of thetwist (i.e., the number of the rotation) is increased too much, thefiber 111 is broken.

It is difficult to form a cylindrical coil having a spring index C ofless than 1 by twisting a fiber formed of a resin other than linearlow-density polyethylene (e.g., low-density polyethylene, high-densitypolyethylene, or nylon 66). This is because the fiber formed of a resinother than linear low-density polyethylene (e.g., low-densitypolyethylene) is broken due to its low durability against the loadgenerated inside by the twist before the spring index C reaches lessthan 1. Alternatively, this is because the fiber formed of a resin otherthan linear low-density polyethylene (e.g., high-density polyethylene ornylon 66) has a spring index C of 1 or more. For more detail, see theexamples and the comparative examples which will be described later.

The present inventors found through experiments that a fiber formed oflinear low-density polyethylene is not broken even if its spring index Cis less than 1.

A typical coil formed of metal may have a spring index C of not lessthan 4 and not more than 22 in light of its performance andmanufacturing easiness. However, in the present embodiment, thecylindrical coil is formed of linear low-density polyethylene and has asmall spring index C of less than 1. The small spring index C of lessthan 1 is required to achieve a high displacement rate which will bedescribed later.

Linear low-density polyethylene may have a density of not less than0.915 g/cm³ and not more than 0.925 g/cm³ and a weight-average molecularweight of not less than 50 kg/mol and not more than 200 kg/mol. Linearlow-density polyethylene is composed of ethylene monomer units eachrepresented by the chemical structural formula —(CH₂—CH₂)_(n)— (where nis a natural number) and a-olefin monomer units each represented by thechemical structural formula —(CH₂CHR)_(m)— (where m is a natural number,and R represents a hydrocarbon group).

The molar ratio of the α-olefin monomer units to the ethylene monomerunits may be not less than 2.5% and not more than 3.5%. In other words,the value of m/n may be not less than 0.025 and not more than 0.035.Each of the a-olefin monomer units may have a carbon number of not lessthan 4 and not more than 8. An example of R is —CH₂CH₃, —CH₂CH₂CH₂CH₃,—CH₂CH(CH₃)CH₃, or —CH₂CH₂CH₂CH₂CH₂CH₃.

(Base 120)

As shown in FIG. 1A, the actuator according to the present embodimentmay comprise a plate-like base 120. The plate-like base 120 comprises afirst protrusion 121 a at one end thereof. The one end of the foldedfiber 111 c is fixed to the first protrusion 121 a through a holdingfixture 122. The plate-like base 120 comprises a second protrusion 121 bat the other end. The other end of the folded fiber 111 c is connectedto one end of a rod 123. The second protrusion 121 b has a through hole121 c. The rod 123 penetrates the through hole 121 c. The rod 123 has ahook 124 at the other end thereof.

A plate-like slider 125 is located slidably on the plate-like base 120between the plate-like base 120 and the one end of the rod 123. Theplate-like slider 125 moves along the coil axis 110LA together with theextension and the contraction of the cylindrical coil formed of thefolded fiber 111 c. More specifically, when the folded fiber 111 c isheated, as shown in FIG. 1B, the plate-like slider 125 also moves alongthe coil axis 110LA. In place of or together with the plate-like slider125, a pulley or a guide tube may be used.

In FIG. 1A, the actuator comprises one fiber 111 c. The actuator maycomprise two or more fibers 111 c. One fiber 111 b may be obtained byintegrally twisting two or more fibers 111 a which are arrangedparallel. One fiber 111 c may be obtained by integrally twisting two ormore twisted fibers 111 b which are arranged parallel.

In order to prevent the twist and the fold of the fiber 111 c fromloosening, it is desirable that one end of the fiber 111 c is fixed. Inother words, it is desirable that the one end of the fiber 111 c isfixed by the folding fixture 122.

(Temperature Regulator 130)

An example of the temperature regulator 130 is a heater or a cooler. Thetemperature regulator 130 may have at least one of the heater and thecooler. The temperature regulator 130 may have both of the heater andthe cooler. An example of the cooler is a Peltier element. Hot water orcold water may be supplied to heat or cool the fiber 111 c.

As shown in FIG. 1A, the temperature regulator 130 may be locatedbetween the fiber 111 c and the plate-like base 120. In this case, thetemperature regulator 130 has a shape of a thin plate. In other words,at least one of a heater and a Peltier element having a shape of a thinplate may be located between the fiber 111 c and the plate-like base120.

As shown in FIG. 2E, the fiber 111 c may be coated with a metal film140. Electric wires 142 a and 142 b are electrically connected to thesurfaces located at one end and the other end of the fiber 111 c,respectively. Electric energy may be supplied through the electric wires142 a and 142 b from a controller 143 which functions as the temperatureregulator 130 to the metal film 140.

The temperature regulator 130 having the metal film 140 may be used incombination with the Peltier element. For example, the metal film 140 isheated by supply of electric energy, and thereby the fiber 111 c isheated. The Peltier element having a shape of a thin plate cools thefiber 111 c. The electric energy required for the Peltier element may besupplied from the controller 143.

(Actuator Operation)

When the cylindrical coil formed of the folded fiber 111 c is heated,the cylindrical coil is contracted along the coil axis 110LA. Morespecifically, when the fiber 111 c is heated, the coil bias angle α, isdecreased. For this reason, the pitch p of the cylindrical coil isdecreased. Compare FIG. 4 which shows the state of the fiber 111 c afterthe fiber is heated to FIG. 3 which shows the state of the fiber 111 cbefore the fiber 111 c is heated. In this way, the folded fiber 111 chaving a shape of a cylindrical coil is contracted along the coil axis110LA. When the fiber 111 c is cooled, the fiber 111 c is extended alongthe coil axis 110LA.

The cylindrical coil formed of the folded fiber 111 c may be heated to atemperature of more than 30 degrees Celsius and not more than 100degrees Celsius. In case of not more than 30 degrees Celsius, since thefiber 111 c is heated insufficiently, the folded fiber 111 c would notbe contracted. In case of more than 100 degrees Celsius, the fiber 111 cmay be melted. It is desirable that the cylindrical coil is heatedwithin a range of not less than 50 degrees Celsius and not more than 90degrees Celsius.

The heated fiber 111 c is cooled to a temperature of not more than 30degrees Celsius. The fiber 111 c may be cooled naturally under roomtemperature. Alternatively, the fiber 111 c may be cooled by the coolersuch as a Peltier element. The above-mentioned heating and cooling maybe repeated.

As demonstrated in the examples which will be described later, thepresent inventors found that the fiber 111 c formed of linearlow-density polyethylene has a high displacement rate DR of not lessthan 0.38×10⁻²/° C., compared to a case where the folded fiber 111 c isformed of another resin.

The displacement rate DR is defined by the following mathematicalformula (I).

(Displacement Rate DR)=(x−y)/(x·ΔT)  (I)

-   -   where    -   x represents a length of the fiber along the axis direction of        the cylindrical coil before the fiber is heated,    -   y represents a length of the fiber along the axis direction of        the cylindrical coil after the fiber is heated, and    -   ΔT represents a temperature difference of the folded fiber        between before and after the fiber is heated.

As just described, when the folded fiber 111 c is formed of linearlow-density polyethylene, the displacement rate DR is a high value of0.38×10⁻²/° C. On the other hand, in case where the folded fiber 111 cis formed of a resin other than linear low-density polyethylene (e.g.,high-density polyethylene or nylon 66), the displacement rate DR is alow value. For example, the fiber 111 c formed of high-densitypolyethylene has a low displacement rate DR of 0.12×10⁻²/° C. The fiber111 c formed of nylon 66 has a low displacement rate DR of 0.096×10⁻²/°C.

EXAMPLES

Hereinafter, the present invention will be described in more detail withreference to the examples.

Inventive Example 1A

L-LDPE pellets (available from Sigma Aldrich Corporation) having adensity of 0.918 g/cm³ were loaded into a melt extruder. While thetemperature inside the melt extruder was maintained at 220 degreesCelsius, the L-LDPE was left at rest in the inside of the melt extruderfor approximately 30 minutes. Then, the melted L-LDPE was pushed out ofthe nozzle attached to the tip of the melt extruder. The nozzle had adiameter of 1 millimeter. In this way, yarn formed of L-LDPE wasobtained from the tip of the nozzle. The yarn was wound around a firstroller (not shown) having a diameter of 5 centimeters. In this way, theyarn formed of L-LDPE (diameter: approximately 0.5 millimeters) wasobtained. This yarn had an elastic coefficient of 0.16 GPa.

Next, one end of the yarn was bound to a second roller (not shown)having a diameter of 5 centimeters. A plate heated to 80 degrees Celsiuswas located between the first roller and the second roller. While theyarn was brought into contact with the surface of the plate, the yarnwas supplied from the first roller and the yarn extended by the heat waswound around the second roller. In this way, the fiber 111 a woundaround the second roller was obtained. In other words, the extended yarnis the fiber 111 a. The rotation speed of the first roller was 2 rpm.The rotation speed of the second roller was 20 rpm. In this way, theyarn was extended to obtain the fiber 111 a. The fiber 111 a had adiameter of 0.12 millimeters. FIG. 5A is a photograph showing the fiber111 a. In this way, the fiber 111 a shown in FIG. 2A was obtained. Thefiber 111 a had an elastic coefficient of 2.5 GPa.

Then, the fiber 111 a was cut to obtain the fiber 111 a having a lengthof 26 millimeters. While a tension was applied to the fiber 111, thefiber 111 was twisted to obtain the twisted fiber 111 b shown in FIG.2B. Furthermore, the fiber 111 was twisted to obtain the folded fiber111 c shown in FIG. 2C. In the inventive example 1A, the tension was 8MPa. FIG. 6A is a photograph showing the folded fiber 111 c obtained inthe inventive example 1A. The length L3 of the folded fiber 111 c was9.7 millimeters.

The folded fiber 111 c obtained in the inventive example 1A had a springindex C of 0.73.

Inventive Example 1B

In the inventive example 1B, an experiment similar to the inventiveexample 1A was conducted, except that the tension was 10 MPa. The foldedfiber 111 c obtained in the inventive example 1B had a spring index C of0.52.

Inventive Example 1C

In the inventive example 1C, an experiment similar to the inventiveexample 1A was conducted, except that the tension was 17 MPa. The foldedfiber 111 c obtained in the inventive example 1C had a spring index C of0.54.

Inventive Example 1D

In the inventive example 1D, an experiment similar to the inventiveexample 1A was conducted, except that the tension was 20 MPa. The foldedfiber 111 c obtained in the inventive example 1D had a spring index C of0.50.

Inventive Example 1E

In the inventive example 1E, an experiment similar to the inventiveexample 1A was conducted, except that the tension was 30 MPa. The foldedfiber 111 c obtained in the inventive example 1E had a spring index C of0.50.

Comparative Example 1A

In the comparative example 1A, an experiment similar to the inventiveexample 1A was conducted except the following matters (I)-(IV).

(I) In place of L-LDPE, used were pellets of low-density polyethylene(hereinafter, referred to as “LDPE”, available from Sigma AldrichCorporation) having a density of 0.906 g/cm ³.

(II) The temperature inside the melt extruder was maintained at 95degrees Celsius.

(III) The temperature of the heated plate was 85 degrees Celsius.

(IV) The rotation speed of the second roller was 8 rpm.

FIG. 5B is a photograph showing the extended fiber 111 a obtained in thecomparative example 1A. This fiber 111 a had a diameter of 0.1millimeter and an elastic coefficient of 0.1 GPa.

While a tension of 5 MPa was applied to the fiber 111, the fiber 111 awas twisted. However, before the fiber 111 was folded, namely, beforethe shape of the cylindrical coil was formed, the fiber 111 a wasbroken. In other words, the fiber 111 a was broken into two parts.

Comparative Example 1B

In the comparative example 1B, an experiment similar to the comparativeexample 1A was conducted, except that the tension was 10 MPa. Similarlyto the case of the comparative example 1A, the fiber 111 a was brokenbefore the shape of the cylindrical coil was formed.

Comparative Example 1C

In the comparative example 1C, an experiment similar to the comparativeexample 1A was conducted, except that the rotation speed of the secondroller was 12 rpm. However, the yarn was cleaved into two parts betweenthe first roller and the second roller. Therefore, the fiber 111 a wasnot obtained.

Comparative Example 2A

In the comparative example 2A, an experiment similar to the inventiveexample 1A was conducted except the following matters (I)-(III).

(I) In place of L-LDPE, used were pellets of high-density polyethylene(hereinafter, referred to as “HDPE”, available from Sigma AldrichCorporation) having a density of 0.96 g/cm³.

(II) The temperature of the heated plate was 100 degrees Celsius.

(III) The rotation speed of the first roller and the second roller was 1rpm and 25 rpm, respectively.

FIG. 5C is a photograph showing the extended fiber 111 a obtained in thecomparative example 2A. This fiber 111 a had a diameter of 0.14millimeters and an elastic coefficient of 1.5 GPa.

Then, the fiber 111 a was cut to obtain the fiber 111 a having a lengthof 70 millimeters. While a tension of 10 MPa was applied to the fiber111, the fiber 111 was twisted to obtain the twisted fiber 111 b shownin FIG. 2B. Furthermore, the fiber 111 was twisted to obtain the foldedfiber 111 c shown in FIG. 2C. FIG. 6B is a photograph showing the foldedfiber 111 c obtained in the comparative example 2A. The length L3 of thefolded fiber 111 c was 13.3 millimeters.

The folded fiber 111 c obtained in the comparative example 2A had aspring index C of 1.21.

Comparative Example 2B

In the comparative example 2B, an experiment similar to the comparativeexample 2A was conducted, except that the tension was 20 MPa. The foldedfiber 111 c obtained in the comparative example 2B had a spring index Cof 1.03.

Comparative Example 2C

In the comparative example 2C, an experiment similar to the comparativeexample 2A was conducted, except that the tension was 30 MPa. In thecomparative example 2C, the fiber 111 a was obtained; however, the fiber111 was broken during the twist before the shape of the coil was formed.

Comparative Example 3A

In the comparative example 3A, an experiment similar to the inventiveexample 1A was conducted except the following matters (I)-(IV).

(I) In place of L-LDPE, used were pellets of nylon 66 (available fromSigma Aldrich Corporation) having a density of 1.14 g/cm³. After thepellets were left at rest in a vacuum oven maintained at 210 degreesCelsius for six hours, the pellets were loaded into the melt extruder.

(II) The temperature inside the melt extruder was maintained at 265degrees Celsius.

(III) The temperature of the heated plate was 175 degrees Celsius.

(IV) The rotation speed of the first roller and the second roller was 5rpm and 25 rpm, respectively.

FIG. 5D is a photograph showing the extended fiber 111 a obtained in thecomparative example 3A. This fiber 111 a had a diameter of 0.12millimeters and an elastic coefficient of 3.7 GPa.

Then, the fiber 111 a was cut to obtain the fiber 111 a having a lengthof 56 millimeters. While a tension of 17 MPa was applied to the fiber111, the fiber 111 was twisted to obtain the twisted fiber 111 b shownin FIG. 2B. Furthermore, the fiber 111 was twisted to obtain the foldedfiber 111 c shown in FIG. 2C. FIG. 6C is a photograph showing the foldedfiber 111 c obtained in the comparative example 3A. The length L3 of thefolded fiber 111 c was 12.1 millimeters.

The folded fiber 111 c obtained in the comparative example 3A had aspring index C of 1.15.

Comparative Example 3B

In the comparative example 3B, an experiment similar to the comparativeexample 3A was conducted, except that the tension was 30 MPa. The foldedfiber 111 c obtained in the comparative example 3B had a spring index Cof 1.1.

Comparative Example 3C

In the comparative example 3C, an experiment similar to the comparativeexample 3A was conducted, except that the tension was 45 MPa. In thecomparative example 3C, the fiber 111 was obtained; however the fiber111 was broken during the twist before the shape of the coil was formed.

Thermomechanical Analysis

The folded fibers 111 c obtained in the inventive example 1A, thecomparative example 2A, and the comparative example 3A were subjected tothermomechanical analysis. More specifically, the folded fibers 111 cwere loaded into a thermomechanical analysis device (available fromRigaku Corporation, trade name: TMA8310) to analyze the thermomechanicalproperties thereof. FIG. 7 is a graph showing the thermomechanicalproperties of the folded fibers 111 c obtained in the inventive example1A, the comparative example 2A, and the comparative example 3A.

In FIG. 7, the horizontal axis indicates temperature (Celsius). Thevertical axis indicates strain. The strain is calculated in accordancewith the following mathematical formula (II).

(Strain)=(x−y)/(x)  (II)

-   -   where    -   x represents a length of the folded fiber along the axis        direction of the cylindrical coil before the folded fiber is        heated (namely, at a temperature of approximately 30 degrees        Celsius), and    -   y represents a length of the folded fiber along the axis        direction of the cylindrical coil after the folded fiber is        heated.

In other words, the following mathematical formula (III) is satisfied.

(Displacement Rate DR)=(Strain)/ΔT

-   -   where ΔT represents a temperature difference of the folded fiber        between before and after the fiber is heated.

In the inventive example 1A, the length L3 of the folded fiber 111 c was9.7 millimeters. When the fiber 111 c was heated to 90 degrees Celsius,the fiber 111 c had a length L3 of 7.5 millimeters. In other words, whenthe fiber 111 c was heated to 90 degrees Celsius, the fiber 111 c wascontracted in such a manner that the fiber 111 c had a length L3 of 7.5millimeters. Then, when the fiber 111 c was cooled to 30 degreesCelsius, the length L3 of the fiber 111 c returned to 9.7 millimeters.

In the inventive example 1A, the displacement rate DR was calculated asbelow.

Displacement rate DR=(9.7 mm−7.5 mm)/(9.7 mm·(90° C.−30°C.))=0.38×10⁻²/° C.

In the comparative example 2A, the length L3 of the folded fiber 111 cwas 13.3 millimeters. Then, the folded fiber 111 c was heated to 90degrees Celsius. In the comparative example 2A, when the fiber 111 c washeated to 90 degrees Celsius, the fiber 111 c had a length L3 of 12.3millimeters. When the fiber 111 c was cooled to 30 degrees Celsius, thelength L3 of the fiber 111 c returned to 13.3 millimeters.

In the comparative example 2A, the displacement rate DR was calculatedas below.

Displacement rate DR=(13.3 mm−12.3 mm)/(13.3 mm·(90° C.−30°C.))=0.13×10⁻²/° C.

In the comparative example 3A, the folded fiber 111 c had a spring indexC of 1.15. The length L3 of the folded fiber 111 c was 12.1 millimeters.Then, the folded fiber 111 c was heated to 90 degrees Celsius. In thecomparative example 3A, when the fiber 111 c was heated to 90 degreesCelsius, the fiber 111 c had a length L3 of 11.4 millimeters. When thefiber 111 c was cooled to 30 degrees Celsius, the length L3 of the fiber111 c returned to 12.1 millimeters.

In the comparative example 3A, the displacement rate DR was calculatedas below.

Displacement rate DR=(12.1 mm−11.4 mm)/(12.1 mm·(90° C.−30°C.))=0.096×10⁻²/° C.

The following Table 1 and Table 2 show the results of the inventiveexamples and the comparative examples.

TABLE 1 Materials of Fiber Tension 111 (MPa) Spring Index C Inventiveexample 1A L-LDPE 8 0.73 Inventive example 1B L-LDPE 10 0.52 Inventiveexample 1C L-LDPE 17 0.54 Inventive example 1D L-LDPE 20 0.50 Inventiveexample 1E L-LDPE 30 0.50 Comparative example 1A LDPE 5 (broken)Comparative example 1B LDPE 10 (broken) Comparative example 1C LDPE(Fiber was not obtained) Comparative example 2A HDPE 10 1.21 Comparativeexample 2B HDPE 20 1.03 Comparative example 2C HDPE 30 (broken)Comparative example 3A Nylon 66 17 1.15 Comparative example 3B Nylon 6630 1.1  Comparative example 3C Nylon 66 45 (broken)

TABLE 2 Length Length L3 L3 (mm) (mm) at 30 at 90 Materials of degreesdegrees Displacement Fiber 111 Celsius Celsius Rate (° C.⁻¹) InventiveL-LDPE 9.7 7.5 0.38 × 10⁻² example 1A Comparative HDPE 13.3 12.3 0.13 ×10⁻² example 2A Comparative Nylon 66 12.1 11.4 0.096 × 10⁻²  example 3A

As is clear from Table 1, the fiber formed of linear low-densitypolyethylene is not broken even when the spring index C is less than 1.However, it is impossible to form a cylindrical coil having a springindex C of less than 1 by twisting the fiber formed of low-densitypolyethylene, high-density polyethylene, or nylon 66.

As is clear from Table 2, the fiber 111 c formed of linear low-densitypolyethylene had a high displacement rate DR of 0.38×10⁻²/° C. On theother hand, the fibers 111 c formed of high-density polyethylene ornylon 66 had low displacement rates DR of 0.12×10⁻²/° C. and0.096×10⁻²/° C., respectively.

INDUSTRIAL APPLICABILITY

The actuator according to the present invention can be used as anartificial muscle.

REFERENTIAL SIGNS LIST

110LA Coil axis

111 Fiber

111 a Fiber which is neither twisted nor folded

111 b Fiber which is twisted, however, not folded.

111 c Fiber which is twisted and folded

111LA Fiber axis

R1 Rotation direction of cylindrical coil

P Pitch of coil

d Diameter of fiber

D Mean diameter of cylindrical coil

D′ External diameter of coil

L1 Length of fiber 111 a

L2 Length of fiber 111 b

L3 Length of fiber 111 c

140 Metal film

142 a Electric wire

142 b Electric wire

143 Controller

α_(c) Coil bias angle

α_(f) Fiber bias angle

120 Base

121 a First protrusion

121 b Second protrusion

121 c Through hole

122 Holding fixture

123 Rod

124 Hook

125 Slider

1. An actuator, comprising: a fiber; and a temperature regulator capableof at least one of heating and cooling the fiber, wherein the fiber istwisted around a longitudinal axis thereof; the fiber is folded so as tohave a shape of a cylindrical coil; the fiber is formed of linearlow-density polyethylene; and the following mathematical formula (I) issatisfied:D/d<1  (I) where D represents a mean diameter of the cylindrical coil;and d represents a diameter of the fiber.
 2. The actuator according toclaim 1, wherein the longitudinal axis of the fiber accords with arotation direction of the cylindrical coil.
 3. The actuator according toclaim 1, wherein the fiber has a density of not less than 0.915 g/cm³and not more than 0.925 g/cm³; the fiber has a weight-average molecularweight of not less than 50 kg/mol and not more than 200 kg/mol; thefiber is composed of ethylene monomer units each represented by thechemical structural formula —(CH₂CH₂)_(n)— (where n is a natural number)and α-olefin monomer units each represented by the chemical structuralformula —(CH₂CHR)_(m)— (where m is a natural number, and R represents ahydrocarbon group); each of the α-olefin monomer units has a carbonnumber of not less than 4 and not more than 8; and a molar ratio of theα-olefin monomer units to the ethylene monomer units is not less than2.5% and not more than 3.5%.
 4. The actuator according to claim 1,wherein one end of the fiber is a fixed end; and the other end of thefiber is extendable along an axis direction of the cylindrical coil. 5.The actuator according to claim 1, further comprising: a base; whereinthe base comprises a protrusion; one end of the fiber is fixed to theprotrusion; and the other end of the fiber is extendable along an axisdirection of the cylindrical coil.
 6. The actuator according to claim 5,wherein the other end of the fiber is provided with at least one of aplate-like slider, a pulley, and a guide tube; and the least one of theplate-like slider, the pulley, and the guide tube is slidable on thebase along the axis direction of the cylindrical coil.
 7. The actuatoraccording to claim 1, further comprising: a base; wherein thetemperature regulator is located between the base and the fiber; and thetemperature regulator has a shape of a plate.
 8. The actuator accordingto claim 1, wherein the temperature regulator comprises a controller anda metal film; the fiber is coated with the metal film; and thecontroller supplies electric energy to the metal film.
 9. The actuatoraccording to claim 8, further comprising: a base; wherein thetemperature regulator further comprises a Peltier element; the Peltierelement is located between the base and the fiber; and the Peltierelement has a shape of a plate.
 10. The actuator according to claim 1,wherein the following mathematical formula (I) is satisfied:(Displacement Rate DR)≧0.38×10⁻²/° C.  (I) where (Displacement Rate DR)is equal to (x−y)/(x·ΔT) where x represents a length of the fiber alongan axis direction of the cylindrical coil before the fiber is heated; yrepresents a length of the fiber along the axis direction of thecylindrical coil after the fiber is heated; and ΔT represents atemperature difference of the fiber between before and after the fiberis heated.
 11. A method for extending and contracting a fiber; themethod comprising: (a) heating the fiber to contract the fiber; whereinthe fiber is twisted around a longitudinal axis thereof; the fiber isfolded so as to have a shape of a cylindrical coil; the fiber is formedof linear low-density polyethylene; the following mathematical formula(I) is satisfied:D/d<1  (I) where D represents a mean diameter of the cylindrical coil;and d represents a diameter of the fiber; and the fiber is contractedalong an axis direction of the cylindrical coil; and (b) cooling thefiber to extend the fiber; wherein the fiber is extended along the axisdirection of the cylindrical coil.
 12. The method according to claim 11,wherein in the step (a), the fiber is heated to a temperature of morethan 30 degrees Celsius and not more than 100 degrees Celsius.
 13. Themethod according to claim 11, wherein in the step (b), the fiber iscooled to a temperature of not more than 30 degrees Celsius.
 14. Themethod according to claim 11, wherein the step (a) and the step (b) arerepeated.
 15. The method according to claim 11, wherein in the step (a),the following mathematical formula (I) is satisfied:(Displacement Rate DR)≧0.38×10⁻²/° C.  (I) where (Displacement Rate DR)is equal to (x−y)/(x·ΔT) where x represents a length of the fiber alongthe axis direction of the cylindrical coil before the fiber is heated; yrepresents a length of the fiber along the axis direction of thecylindrical coil after the fiber is heated; and ΔT represents atemperature difference of the fiber between before and after the fiberis heated.